1. Field of the Invention
The present invention relates to a circuit board tester, and more particularly, to a tester for testing conduction and insulation between optional pads of a circuit board in a noncontact manner.
2. Description of the Related Art
Circuit boards increasingly have a large number of printed conductive wirings (which may be called as traces or nets, and hereinafter referred to as traces) and a large number of pads connected to the traces. This increases the time required for testing conduction and insulation among the pads. Contact testers must prepare contact probes suitable for a circuit board to be tested and control the contact probes on test pads of the circuit board. When the pads are very small, it is difficult to form an array of contact probes suitable for the pads and simultaneously and surely bring the probes in contact with the pads. It is necessary, therefore, to provide a noncontact tester to speedily test a circuit board for conduction and insulation among pads thereof. In particular, it is required to provide a tester for testing a high-density circuit board such as a multichip module (MCM) board for conduction and insulation among pads thereof.
Circuit board testing techniques are disclosed in, for example, Japanese Unexamined Patent Publication Nos. 3-295476, 3-118484, and 4-236367.
The Publication JUPP 3-295476 (first prior art) discloses a contact tester employing a test head in which many metal contact probes are embedded. The probes simultaneously come into contact with many pads on a circuit board. A signal is applied to two arbitrary pads through the corresponding probes, and a voltage or current between the probes is measured to calculate resistance between the two pads. According to the calculated resistance, it is determined whether or not conduction between the pads is allowable if the pads are on the same trace, or whether or not insulation between the pads is allowable if the pads are on different traces.
This technique is difficult to apply to circuit boards involving very fine wiring and pads, or a great number of pads. Accordingly, a flying probe technique and a two- or four-probe technique that separately move two to four metal probes to successively measure resistance between every pair of pads, have been developed.
The Publication JUPP 3-118484 (second prior art) discloses a noncontact tester. This tester emits an electron beam to charge an arbitrary pad and a trace connected to the pad to a given voltage, to see whether or not the voltage appears on another pad or trace. If the voltage appears on another pad on the same trace, it is determined that conduction is good. If pads on another trace maintain the same voltage level before and after the charging, it is determined that insulation is good.
The JUPP 4-236367 (third prior art) discloses a noncontact tester employing a laser beam and a photoconductive sheet having a transparent conductive film. The tester emits a laser beam so that the conductive film is electrically connected to a test pad through a part of the photoconductive sheet where the laser beam irradiates, to thereby charge the test pad. Thereafter, the tester emits a laser beam to measure charges at another pad through the conductive film, thereby determining the quality of conduction and insulation between the two pads.
These prior arts have problems, however, as described below.
According to the first prior art, it is nearly impossible to fabricate a test head having an array of probes to deal with several thousands to several tens of thousands of pads of a high-density circuit board with the pads each extending several tens of micrometers square and being arranged at pitches of about 10 micrometers. It is impossible to correctly bring the probes into contact with the pads. Instead of an array of probes, four discrete probes may be employed to surely make contact with pads and apply and measure voltages. This technique, however, takes a very long time for testing. When testing 2000 pads, approximately 500 hours will be needed to carry out 2000.times.2000 insulation tests because the probes need at least 0.5 seconds to measure resistance between each pair of the pads.
The second prior art employing electron beams may not have this kind of problem but it has another problem. The size of a circuit board is usually 10 to several tens of centimeters square, so that the tester must be three to four meters square to accommodate the circuit board in a vacuum chamber. In the vacuum chamber, the circuit board requires a long degassing time, so that it takes about one hour to start the test.
According to the third prior art, the photoconductive sheet and test pads must be completely in contact with each other. This is very difficult because the circuit board has irregularities of several micrometers.
The second and third prior arts charge an optional test pad and observe a charged state at another pad due to leakage from the charged pad. The second and third prior arts have a principle problem that it is difficult to measure correct resistance between the pads because charge and discharge time constants are affected not only by the resistance but also by the electrostatic capacitance of traces on which the pads are contained.